1. Field of the Invention
The present invention relates to composite metallic ultrafine particles and a process for producing the same, and more particularly to composite metallic ultrafine particles which have excellent dispersion stability and can be produced on an industrial scale, and a process for producing the same, and a method and an apparatus for forming an interconnection with use of the same.
2. Description of the Related Art
Generally, as a process for producing metallic ultrafine particles having a diameter of not more than 100 nm, there has been known a process in which metal is evaporated under vacuum in the presence of a small amount of gas, and then metallic ultrafine particles are condensed from the vapor phase to obtain ultrafine metallic particulates. However, this physical process suffers from practical problems that {circle around (1)} the variation in the particle diameter distribution is so large that even if a heating process is performed for film formation, grain boundaries are left, and a uniform metallic film cannot be obtained, {circle around (2)} the amount of metallic ultrafine particles produced in a single operation is so small that this process is not suitable for mass production, and {circle around (3)} devices for an electron beam, plasma, laser, induction heating, or the like are necessary for the evaporation of metal, and hence the production cost rises, for example.
Further, when the metallic ultrafine particles are solely taken out into the air, they are agglomerated. Therefore, it is necessary to disperse the particles in a solvent with use of a surface-active agent or the like. However, since the dispersion stability is poor, the storage stability is unsatisfactory.
There has been reported a method for producing metallic ultrafine particles in which metal ions produced from a metallic salt in an aqueous solvent are stabilized by a polymeric protective agent. However, such metallic ultrafine particles are limited to being handled in an aqueous system and thus have poor flexibility. Further, in this case, a high-molecular weight dispersant should be used for stabilization of metallic ultrafine particles. Therefore, metal content is lowered, and the ultrafine particles have little expectation of using as a metal source.
Metallic ultrafine particles have high activity and are unstable. Therefore, when such metallic ultrafine particles are gathered in a bare particle state, the particles are easily adhered to each other to cause an agglomeration or to be chained. In order to stabilize metallic ultrafine particles in such a state that the particles are separated from each other, it is necessary to form a certain protective coating on the surface of the metallic ultrafine particles. Further, in order that metallic ultrafine particles are stable as particles even with a high metal content, a metallic core should be stably bonded to the protective coating formed therearound.
When the production of metallic ultrafine particles on an industrial scale is taken into consideration, metallic ultrafine particles should preferably be safety and simply produced in a production process, and such metallic ultrafine particles are required to be utilized in various fields and to have good flexibility.
The present inventors have found the following thing. A certain metallic salt, a metallic oxide, or a metallic hydroxide is mixed with an organic compound including a functional group having chemisorption capability onto a metal produced from the metallic salt, the metallic oxide, or the metallic hydroxide. In a process in which the mixture is heated under the reflux condition of the organic compound for reaction, a core of an ultrafine particle of pure metal is produced by pyrolysis of the metallic salt, the metallic oxide, or the metallic hydroxide. The organic compound is chemisorbed onto the core by the functional group of the organic compound to form composite metallic ultrafine particles having a stable protective coating with high efficiency.
Further, it has been found that since a metallic salt, a metallic oxide, or a metallic hydroxide as a metal source and an organic compound for a protecting coating are different from each other, there is a possibility that the metal content, the reactivity, the particle diameter, or the like of the composite metallic ultrafine particles to be produced can be controlled by varying the combination of the metal source with the organic compound. Further, since the amount ratio of the metal source to the organic compound can also be manipulated as desired, processes from synthesis to purification of the composite metallic ultrafine particles can easily be optimized.
The present inventors have found that, in a process in which a certain metallic salt is mixed with an organic compound including an alcoholic hydroxyl group and pyrolyzed, an alcohol is bonded to the periphery of a core metal with a metal alkoxide bond to form composite metallic ultrafine particles having a stable protective coating. Further, the present inventors have invented a novel method for forming an interconnection on a semiconductor substrate with use of this composite metallic ultrafine particle, and an apparatus using this method.